The Earth's core is composed mainly of iron, divided into a solid inner center roughly 1,500 miles wide covered by a liquid outer layer about 1,400 miles thick. Even though the bulk of the core is iron, researchers also knew it contained a small amount of lighter elements such as oxygen and sulfur. As the inner core crystallized over time, scientists think this process forced out most of these light elements, which then migrated through the liquid outer core.

Now geoscientists think they have detected all these light elements concentrated in the outermost parts of the core.

"Ever since core structure started to be studied, there were hints of structure there — that's why we looked for it," said researcher George Helffrich, a geologist and seismologist at the University of Bristol in England.

Seismic speed changesTo investigate the core, researchers monitored seismic waves that traveled through its outer layer. The waves were generated by earthquakes in South America and the southwestern Pacific Ocean, and were recorded using arrays of seismometers in Japan and northern Europe.

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The speeds at which seismic waves traveled through the outer core at different depths suggest that its composition does not remain the same all the way through. Instead, the uppermost 185 miles or so is a distinct structure, with the section nearest the boundary consisting up to 5 percent by weight of light elements.

"The seismic structure we found is hard to deny — the signal is obvious to the eye in the data that we used," Helffrich told OurAmazingPlanet.

While Helffrich would not call their discovery a new layer of the core, he noted that "others might." He likened the potential new layer to the layers of the atmosphere.

"Think about the stratosphere above your head — is it a layer? There's no boundary to it, only a change in the temperature profile with altitude," Helffrich said. "Similarly, there's no boundary we infer inside the top of the core, just a slow droop in wave speed and then, possibly, a slight increase as you approach the core's surface."

"One enduring problem is how to power it for the 3 billion years, for which it seems to have been running," Helffrich said. The spinning of the Earth's core is thought to power the magnetic field that surrounds the planet.

The most plausible solution, Helffrich explained, is that expulsion of light elements from the inner core liberated what is called gravitational potential energy. As this lighter liquid rose upward, it imparted energy downward that drove the flow of metal in the core, which in turn helped keep the magnetic field running. "That fits with the wave speed profile that we observe," Helffrich said.

Future earthquakes might provide even better looks at this outermost structure. "New seismic networks in, say, China, India or the U.S. might provide even larger datasets too," Helffrich said. "I suspect that the main improvement to the work will be to model core liquids better and to balance the growth of the inner core with the composition of the layer."

Helffrich and his colleague, geophysicist Satoshi Kaneshima at the University of Kyushu in Japan, detailed their findings in the Dec. 9 issue of the journal Nature.